In surface drilling, the drill bit at one end of a drill stem is rotated and used to bore into the earth. The drill stem and extensions referred to as the "drill string" are typically hollow. During drilling, gas or fluid is pumped down through the drill stem, the gas or fluid then rises to the surface through the annular space between the drill stem and the wall of the hole bored by the drill stem. The hole bored by the drill stem is referred to as the "bore hole". The gas or fluid may comprise air, nitrogen, water, foam, drilling mud, or any other substance that is capable of removing cuttings from the bottom of the bore hole to the surface of the well. For example, in a conventional drilling rig, drilling mud is used to cool the drill bit and remove cuttings from the bottom of the drill hole, by carrying them to the surface in the annular space between the drill string and the bore hole wall. Traditionally the drilling mud coming from the well bore was dumped into a reserve pit. The drilling mud would then be allowed to settle so that sediments fell to the bottom of the pit. The drilling mud could then be pumped from the top of the pit back into the well bore. However, because of ever increasing environmental concerns many governments have banned the use of in-ground reserve pits. Similarly, in the case of pure air drilling the cuttings and dust returning from the bore hole would be allowed to dissipate in the environment, creating a dust cloud that could extend several kilometers and this is no longer environmentally acceptable.
Today, in place of in-ground reserve pits, tanks are used. The drilling fluid is slowly circulated through the tanks to allow gases to dissipate and solids to settle out. Recent drilling mud circulation systems have used a vibrating screen assembly known as a "shale shaker" to separate out the bulk of the cuttings which drop from the shaker onto the ground or into a pit. Other methods of separation include centrifuge separators to remove the solids while retrieving the substantial portion of the fluid. Once passed the initial separation stage the fluid then enters one or more rectangular open topped "mud tanks". The fluid slowly moves through the mud tank and most of the fine solids which remain suspended after screening or centrifuging settle out. Often the mud tank has one or more transverse weirs or baffles, which divide the tank chamber into compartments. The weir functions to trap settling fine solids and thick mud, allowing "cleaned" mud to advance and to provide tanks with mud at different concentrations. The cleaned mud is then recycled to the well bore.
Such systems work well with relatively liquid drilling fluids. However, in air or combined air and foam drilling systems these separation units are not terribly efficient as the "drilling mud" contains more air and foam than it does fluid or mud. Further, being less dense than conventional drilling mud the output from the bore hole is at considerably higher velocity, which can cause failures in conventional filtration systems.
U.S. Pat. No. 5,718,298 discloses a separation system for use with wells drilled using air or mist as the drilling fluid. The invention as disclosed contains a longitudinal separator tube through which drilling fluid from a blooie line passes. The longitudinal tube contains plurality of openings into which water may be injected as well as a plurality of dump gates through which drilling materials drop into a large receiving tank. A series of angle baffles are utilized within the separator tube to reduce the velocity of the exhaust from the blooie line. When drilling with dense fluids or in the case of large drill cuttings, the separator tube and its associated outlet ports may rapidly become plugged. Should the injection of water into the separator tube be unable to clear any obstructions, then it may be quite difficult to clear out the separator tube.
Thus, there is a need for a drilling fluid separator that is able to separate efficiently and cleanly drill cuttings from a gas, water and foam drilling fluid.